It's great to be ambitious in your subject matter as you'll do better work drawing something you are excited about as opposed to something you find mundane or tedious. But having said that, don't knock the value of just sketching very simple shapes - Cubes, Cylinders, Spheres, etc.
Taking a little time at the beginning of your sketch session to bust out a few simple objects can help get you in sketch mode by letting you focus on the style of the drawing rather than the subject matter.
Theres no shame in going back to basics once in a while.
Just my $0.02 - hope it helps.
"See, how it works is, the train leaves and not the station"